The present invention generally relates to methods and systems for rapidly measuring relative angular alignment of flat surfaces and, more particularly relates to methods and systems for rapidly measuring relative angular alignment of flat machined surfaces.
Currently, coordinate measuring machines (CMM) are used to measure the relative alignment of machined surfaces. However, these devices are very slow and may require hours to measure a complex part such as an engine head. In the time required to measure one part, hundreds of defective parts may be produced by the machining system before results of the measurements are available and the error is corrected.
Alternatively, surfaces can be measured using custom-made gauging systems designed to inspect a specific part. These devices are expensive, require long lead times to produce and can only be used for the specific part for which they have been designed. Measurements using such gauging systems generally take several minutes to perform.
Optical instrumentation for performing multiple simultaneous measurements of surface features of an object has been developed by PERCEPTION CORP. Such instrumentation is be used to identify defects in automobile body assembly. The root cause of a defect is then identified using a stream of variation analysis. Each optical sensor images a line of light on a critical surface feature of the vehicle body and measures the contour of the surface by triangulation. Up to 100 such sensors may be used to perform simultaneous measurements over the body of the vehicle. The accuracy of these measurements is about 100 xcexcm, which is at least an order of magnitude less than the accuracy required for inspecting machined parts.
Optical systems that measure small displacements of machine tools are manufactured by API. Other systems that measure machine errors directly using interferometry are manufactured by ZYGO and HEWLETT PACKARD. OG TECHNOLOGY makes a system that can obtain optical profiles of flat parts from which some machine errors could be inferred. Other manufacturers of optical instrumentation for machining exist, but their instrumentation does not perform the types of measurements required to rapidly obtain precise angular information for machined surfaces.
Techniques for measuring parallelism, perpendicularity and angular alignment can be found in the literature. Some of these include an optical CMM, interferometric techniques, analysis of multiple images from an electronic camera, rotation of an object about a laser beam with measurement of the beam at multiple locations, and various hard gauges. Specific U.S. patents which show these techniques include:
U.S. Pat. No. 4,969,744 Optical angle-measuring device (interferometry);
U.S. Pat. No. 5,430,539 Method and arrangement for checking alignment of body axes for parallelism (rotation about a laser beam);
U.S. Pat. No. 5,774,210 Perpendicularity measuring method and apparatus thereof (analysis of multiple images of an object);
U.S. Pat. No. 5,489,986 Position detecting apparatus (uses two intersecting beams to determine position using interference);
U.S. Pat. No. 5,825,666 Optical coordinate measuring machines and optical touch probes squareness gauge (hard gauge);
U.S. Pat. No. 3,681,849 Squareness gauge (hard gauge); and
U.S. Pat. No. 3,716,920 Precision square (hard gauge).
None of the techniques described in these patents, however, are directed toward the simultaneous measurement of multiple surfaces of a machined part.
Some high precision angular measurement techniques are only useful for very small angles, on the order of a degree. One paper describing how interferometry can be used for this is proposed in xe2x80x9cInterferometric Measurement of Anglesxe2x80x9d by D. Malacara and O. Harris in APPLIED OPTICS 9, 1630-1633 (1970).
A technique that employs total internal reflection to produce an angle dependent phase difference between two directions of polarization that can be detected as a phase shift can be used for a slightly larger range of a few degrees. This is described in xe2x80x9cAngle Measurement Using Total Internal Reflection Heterodyne Interferometryxe2x80x9d by Ming-Horng Chiu and Der-Chin Su in OPTICS ENGINEERING 36(6) 1750-1753 (June 1997).
The traditional way of measuring arbitrary angles is with a CMM. CMMs are produced by a number of different manufacturers and their use is described in their manuals.
Optical CMMs use imaging cameras and triangulation to obtain surface profiles. This instrument is used in stamping to get surface profiles before and after stamping. Before stamping a grid is drawn on the part, and the distortion of the grid as a result of stamping is determined by triangulation calculations. This technique is less accurate than traditional contact CMMs and would be inappropriate for a machining application.
As noted above, correct alignment of flat surfaces is an important aspect of the quality of machined parts. Despite the above-noted prior art, a technique is needed to rapidly and accurately measure parallelism, perpendicularity and relative angular alignment of machined surfaces to determine whether they are within tolerances.
An object of the present invention is to provide an optical method and system for rapidly measuring relative angular alignment of flat surfaces with a particular application to the angular alignment of flat surfaces of machined parts.
Another object of the present invention is to provide an optical method and system for rapidly measuring relative angular alignment of flat surfaces wherein the system is reconfigurable so that the surfaces of many different shaped parts can be measured.
Still another object of the present invention is to provide an optical method and system for rapidly measuring relative angular alignment of flat surfaces to reduce ramp-up time of machining systems, maintain machining systems in calibration, reduce down time, and reduce scrap.
In carrying out the above objects and other objects of the present invention, an optical method for rapidly measuring angular alignment of a flat first surface relative to a flat second surface is provided. The method includes directing a first beam of controlled light at the first surface to generate a corresponding reflected first light signal and directing a second beam of controlled light at the second surface to generate a corresponding reflected second light signal. The method also includes receiving the reflected first and second light signals with an optical component for creating first and second spots of light, respectively, in a detector plane. The method further includes measuring position of radiant energy in the first and second spots of light in the detector plane to produce surface measurement signals which represent angular alignment of the first surface relative to the second surface.